Wireless communication systems are widely deployed to provide various types of communication to mobile users such as voice, data, etc. Mobile user equipments (UEs) typically transition from one wireless system to another wireless system depending on their mobility and the availability of coverage by the wireless systems. For example, transitions can occur between second generation (2G) and third generation (3G) wireless systems, between long term evolution (LTE) and 3G wireless systems or between LTE and Global System for Mobile Communications (GSM) wireless systems. Taking one example, 2G wireless systems typically provide basic digital voice and low rate data services to user equipment (UE) over a broad coverage area. That is, the 2G wireless systems typically have ubiquitous coverage. Broad coverage area is implemented using a plurality of cells, each with an access node (e.g. base station) to provide a wireless access connection between a UE, which is mobile within the coverage area, and the wireless communication system. Multiple access refers to the property of allowing users to have simultaneous or near-simultaneous wireless access to a network over a shared communications medium. The wireless access connection may employ space division multiple access (SDMA), frequency division multiple access (FDMA), time division multiple access (TDMA), code division multiple access (CDMA) and/or orthogonal frequency division multiple access (OFDMA) to allow a plurality of UEs to access the wireless communication system. In one example, the 2G wireless system is based on Global System for Mobile Communications (GSM)/General Packet Radio Service (GPRS)/Enhanced Data Rates for GSM Evolution (EDGE) while the 3G wireless system is based on Universal Mobile Telecommunication System (UMTS). A system may be designed to support one or more standards such as the standard offered by a consortium named “3rd Generation Partnership Project” referred to herein as 3GPP, IS-95, cdma2000, IS-856, W-CDMA, TD-SCDMA, WiMax, Ultra Mobile Broadband (UMB) and other standards.
Many wireless communication systems are upgrading their infrastructure to provide enhanced communication services, such as high rate data services and Internet protocol (IP) packet transport services to mobile UEs. These enhanced communication services are typically provided by 3G wireless systems. In many cases, the 3G wireless systems are implemented only in portions of the broad coverage area provided by 2G wireless systems. That is, in many cases 3G wireless systems do not provide ubiquitous coverage. 3G coverage areas are typically situated in high density population areas, such as the centers of urban areas, airports, shopping centers, business parks, etc. In this case, 3G coverage areas appear as islands of coverage within the broader 2G coverage areas. This diversity of coverage areas introduces the necessity of transitioning the wireless access connection of the mobile UE between the 2G coverage area and 3G coverage area. Although the example of transitioning between the 2G coverage area and 3G coverage area is discussed here, the UE may transition between any coverage areas of any radio access technologies employed by any wireless systems, including but not limited to, UMTS (universal mobile telecommunication system), GSM (Global System for Mobile communications), GSM/GPRS (General Packet Radio Service /EDGE (Enhanced Data Rates for GSM Evolution), LTE (long term evolution), IS-95 (interim standard 95), CDMA2000, EVDO (evolution data optimized) or UMB (ultra mobile broadband), etc. And, during transitioning, services may be dropped if the transitioning procedure is delayed.